Semiconductor memory device

Abstract: A memory cell array is divided into a plurality of blocks selected by an upper row address and further divided into a plurality of sub blocks in a word line direction. Word line driving circuits are provided individually for each of the sub blocks. Further, a plurality of word line driving current supplying circuits are provided which are selectively activated in response to an input of all or part of an upper row address for selection of one of the plurality of blocks to supply word line driving current to the word line driving circuits. The word line driving circuits in the same column direction are supplied with word line driving current by way of at least two signal lines.

Description: Background of the Invention Field of the Invention This invention relates to a semiconductor memory device, and more particularly to a semiconductor memory device which includes a word line driving circuit of the divisional decoding type. Description of the Prior Art The degree of integration of semiconductor memory devices increases with advances in the technology in fine working technique. In particular, high integration has been achieved with dynamic random access memories (hereinafter referred to as DRAMS) which have a simple memory structure, and at present, DRAMs of 16 Mbits are being mass-produced and sample shipment of DRAMs of 64 Mbits is also planned. 64-Mbit DRAMs are semiconductor memory devices of the divisional decoding type wherein word lines are divided into a plurality of blocks and a word line driving circuit for supplying current is provided for each block of word lines to achieve high speed driving of the word lines. Fig. 1 is a circuit diagram showing the basic construction of a semiconductor memory device of the divisional decoding type, and Fig. 2 is a circuit diagram of a word line driving circuit represented by WD1a in Fig. 1. A memory cell array is divided into a plurality of blocks each selected by an upper row address, and each block is divided into a plurality of sub blocks SB1a to SB1c, SB2a to SB2c and SB3a to SB3c in the word line direction. Word line driving circuits WD1a to WD1d are disposed in a manner such that sub block SB1a, SB1b, SB1c lie between word line driving circuit WD1a and WD1b, WD1b and WD1c, WD1c and WD1d, resectively. Word line driving circuits WD2a to WD2d, WD3a to WD3d and WD4a to WD4d are disposed with respect to sub block SB2a to SB2c, SB3a to SB3c and SB4a to SB4c in the same way as word line driving circuits WD1a to WD1d. Word lines in sub block SB1a are alternately connected with word line driving circuit WD1a and WD1b. Similarly, word lines in sublock SB1b, SB1c, ... , SB4c are alternately connected with word line driving circuit WD1b and WD1c, WD1c and WD1d, ... , WD4c and WD4d, respectively. Further, sensing amplifiers SA1a to SA1c, SA2a to SA2c, SA3a to SA3c and SA4a to SA4c for amplifying data of bit lines are provided for sub blocks SB1a to SB1c, SB2a to SB2c, SB3a to SB3c and SB4a to SB4c, respectively. Sensing amplifiers SA1a to SA1c, SA2a to SA2c, SA3a to SA3c and SA4a to SA4c are determined by upper row address signals S1 and S2 and are selectively activated by block select signals BLS1 to BLS4 outputted from block select signal generation circuit BSG1. Memory cells MC0 and MC1 are disposed at points at which word lines WL0a and WL1a intersect with bit lines BL00 and BL10, respectively. In order to select word line driving circuits WD1a to WD1d, complementary signals XSW and XSWB are outputted from row decoder RD1 to word line driving circuits WD1a to WD1d. Meanwhile, word line driving current supplying signals RA01 and RA11 supplied from word line driving current supplying circuit RAG run vertically with respect to word lines WL0a and WL1a on word lines WD1a to WD4d, and word line driving current supplying signal RA00 is inputted to word line driving circuits WD1a, WD2a, WD3a, WD4a, WD1c, WD2c, WD3c, WD4c while word line driving current supplying signal RA11 is inputted to word line driving circuits WDlb, WD2b, WD3b, WD4b, WD1d, WD2d, WD3d and WD4d. Row decoders RD2, RD3 and RD4 are provided corresponding to word line driving circuits WD2a to WD2d, WD3a to WD3d and WD4a to WD4d, respectively. It is to be noted that, while, in the drawings, almost all of output signals of the row decoders, word lines, bit lines, memory cells and so forth are omitted for simplification of the drawings, each block has a similar construction. Word line driving circuits WD1a to WDld, WD2a to WD2d, WD3a to WD3d and WD4a to WD4d are each constituted from a driving circuit of the self-booting type as shown in FIG. 2. If it is assumed that complementary signal XSW and word line driving current supplying signal RA01 are selected by a row address, then N-type transistor Q3 is put into an off-state by complementary signal XSWB. Consequently, the potential at node 1 rises from the ground potential to a potential equal to the difference between the power source voltage and the threshold level voltage of N-type transistor Q2. Then, when the potential of word line driving current supplying signal RA01 rises, the potential at node 1 rises due to self booting of the transistor to a potential higher than word line driving current supplying signal RA01, and the potential at word line WL0a rises to a potential equal to the potential of word line driving current supplying signal RA01. In this instance, not only in word line driving circuit WD1a but also in word line driving circuit WD1c, the potentials at word lines WL0b and WL0c rise so that memory cell on the word line as represented by MCO is selected. Data of the selected memory cell is outputted to the bit line represented by BL00, and sensing amplifiers SA1a to SA1c are activated by block select signal BLS1 to amplify the data. With the circuit construction described above, the wiring line distance per word line is decreased, and the rising rate of the word lines can be increased. The above-described circuit construction, however, has the following disadvantages: Generally, the word line rising time depends upon the wiring line resistance and the load capacitance of the word line and the wiring line resistance and the load capacitance of the word line driving current supplying circuit. In particular, if the wiring line resistance and the load capacities of the word line and the word line driving current supplying circuit are increased, then the word line rising time lengthens. On the other hand, the current consumed in driving the word line increases in proportion to the sum of the load capacities of the word line and the word line driving current supplying signal. However, the wiring line resistance and the load capacitance of the word line depends upon such parameters as the material forming the word line and the number of memory cells connected to the word line, and in addition, the number of word lines to be activated at a time depends upon factors such as the limitation of the refresh cycle. Accordingly, it is difficult to reduce the wiring line resistance and the load capacitance of the word line. On the other hand, with regard to the word line driving current supplying signal, the load capacitance is the sum of the wiring line capacitance and the diffused layer junction capacitance of the word line driving circuit to which the word line driving current supplying signal is inputted. While the wiring line capacitance depends upon the width, the thickness, and the length of the wiring line, width and the thickness cannot be greatly decreased because so doing would increase the resistance value. On the other hand, the wiring line cannot be shortened because of restrictions in terms of layout. Accordingly, in order to reduce the load capacitance of the word line driving current supplying signal, the diffused layer capacitance of the word line driving circuit should be decreased, that is, the number of word line driving circuits to be connected to one word line driving current supplying signal should be decreased. In the conventional word line driving system based on the divisional decoding system described above, however, since a word line driving current supplying signal is inputted in common to word line driving circuits in a block selected by an upper row address, when the number of blocks increases as the storage capacity increases, or when the number of word lines per one block increases, the load capacitance of the word line driving current supplying signal also increases, possibly resulting in a drop in speed when driving word lines or in an increase of current consumption. DE-A-42 43 903 shows in figure 3 a device similar to that shown in figure 1 of the present application, but not having a block select signal generation circuit. DE-A-4 117 881 discloses a memory in which word line driving current supplying circuit means each drive a subset of those word line driving circuit means arranged along the same column. Summary of the Invention It is an object of the present invention to provide a semiconductor memory device wherein a word line rises at a high speed and the current consumption is low even when the storage capacity is increased. In accordance with the present invention, there is provided a semiconductor memory device as defined by claim 1. Since word line driving current is supplied to the word line driving circuits in the same column direction by way of at least two signal lines, the number of word line driving circuits per word line driving current supplying signal is decreased. In other words, the load capacitance of the word line driving current supplying circuits are decreased. Consequently, the word line rising speed is increased, and the current consumption upon driving of the word lines is decreased. The objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention. Brief Description of the Drawings Fig. 1 is a circuit diagram showing the basic construction of a conventional example of a semiconductor memory device; Fig. 2 is a circuit diagram of a word line driving circuit represented by WD1a in Fig. 1; Fig. 3 is a circuit diagram showing the basic construction of a semiconductor memory device according to a first embodiment of the present invention; and Fig. 4 is a circuit diagram showing the basic construction of a semiconductor r memory device according to a second embodiment of the present invention. Description of the Preferred Embodiments Fig. 3 is a circuit diagram showing the basic construction of a first embodiment of the present invention. In the present embodiment, word line driving current supplying circuits RAG1 to RAG4 are provided for sub blocks SB1a to SB1c, SB2a to SB2c, SB3a to SB3c and SB4a to SB4c, respectively. The word line driving current supplying circuits RAG1 to RAG4 are selected by two bits of an upper row address, and the same signals BLS1 to BLS4 inputted as block select signals to sensing amplifiers SA1a to SA1c, SA2a to SA2c, SA3a to SA3c and SA4a to SA4c are inputted to word line driving current supplying circuits RAG1 to RAG4, respectively. In particular, word line driving current supplying circuit RAG1 corresponds to sub blocks SB1a to SB1c; word line driving current supplying circuit RAG2 corresponds to sub blocks SB2a to SB2c; word line driving current supplying circuit RAG3 corresponds to sub blocks SB3a to SB3c; and word line driving current supplying circuit RAG4 corresponds to sub clocks SB4a to SB4c, and one of word line driving current supplying circuits RAG1 to RAG4 is selected and activated in response to activated sub blocks. Word line driving current supplying circuit RAG1 supplies word line driving current supplying signal RA01 to word line driving circuits WD1a and WD1c and supplies word line driving current supplying signal RA11 to word line driving circuits WD1b and WD1d. Word line driving current supplying circuit RAG2 supplies word line driving current supplying signal RA02 to word line driving circuits WD2a and WD2c and supplies word line driving current supplying signal RA12 to word line driving circuits WD2b and WD2d. Word line driving current supplying circuit RAG3 supplies word line driving current supplying signal RA03 to word line driving circuits WD3a and WD3c and supplies word line driving current supplying signal RA13 to word line driving circuits WD3b and WD3d. Word line driving current supplying circuit RAG4 supplies word line driving current supplying signal RA04 to word line driving circuits WD4a and WD4c and supplies word line driving current supplying signal RA14 to word line driving circuits WD4b and WD4d. In the present embodiment, the number of word line driving circuits connected to word line driving current supplying signals is 1/4 that of the conventional example, and as a result, the load capacitance of the word line driving current supplying signal is reduced, thereby enabling reduction of the word line rising time and reduction of the current consumption upon rising of the word line. For example, if it is assumed that 30% of the load capacitance upon rising of a word line is conventionally occupied by the load capacitance of a word line driving current supplying circuit, according to the present embodiment, this 30% portion of the load capacitance can be reduced by 3/4, and consequently, the overall load capacitance can be reduced by about 23%. Since the current consumption increases in proportion to the load capacitance to be charged and discharged, the current consumption upon rising of a word line can be reduced by an equal 23%. Further, since the entire time constant is reduced by 23%, the word line rising time is reduced by a similar amount, although the actual amount of reduction remains somewhat dependent on such factors as how the load is dispersed by the wiring line layout. For simplification of the drawings, the number of blocks to be selected by an upper row address has been limited to four in the present embodiment, the present invention can naturally be applied to cases wherein the number of blocks is five or more. Further, in the present embodiment, while the bit size of an upper row address for selection of a block is equal to the bit size of a row address to be inputted to a word line driving current supplying circuit, they naturally need not be equal to each other, and the numbers may be determined suitably based on a trade-off between an increase of the word line rising speed and an increase in the chip area caused by an increase of the number of word line driving current supplying circuits. FIG. 4 is a circuit diagram showing the basic construction of a second embodiment of the present invention wherein the bit size of an upper row address for selection of a block (activation of a sensing amplifier) and the bit size of an upper row address to be inputted to a word line driving current supplying circuit are made different from each other. The present embodiment includes word line driving current supplying circuit RAG11 for supplying word line driving current supplying signal RA01 to word line driving circuits WD1a, WD2a, WD1c and WD2c and supplying word line driving current supplying signal RA11 to word line driving circuits WD1b, WD2b, WD1d and WD2d, and word line driving current supplying circuit RAG12 for supplying word line driving current signal RA02 to word line driving circuits WD3a, WD4a, WD3c and WD4c and supplying word line driving current supplying signal RA12 to word line driving circuits WD3b, WD4b, WD3d and WD4d. In the present embodiment, the number of word line driving current supplying circuits is equal to one half the number in the first embodiment, and outputs BLS12 and BLS22 of block select signal generation circuit BSG2, which are inputted to the word line driving current supplying circuits RAG11 and RAG12 to select a word line driving current supplying circuit, are generated only by an upper row address signal S1. Since the construction described above can reduce the area occupied by the word line driving current supplying circuits and the area occupied by the word line driving current supplying signals, this is an effective means where it is desired to limit an increase of the chip area. Further, the following connection is effective between a word line driving current supplying signal and the word line driving circuits of each block. In particular, in a case wherein n blocks are provided and one bit of a row address is inputted as a selection signal to the word line driving current supplying circuits as described above, the connection should be such that, for example, when the inputted address of one bit is 0, the word line driving current supplying signal to be activated is connected to the 1st, 3rd, 5th, ... and n-lth blocks, whereas the word line driving current supplying signal to be activated is connected to the 2nd, 4th, 6th, ... and nth blocks when the address is 1. In other words, the blocks are connected such that those blocks which are activated when the address is 0 and those block which are activated when the address is 1 are disposed alternately. In this instance, the load capacitance of each word line driving current supplying signal is divided and disposed substantially uniformly over the wiring line length, and accordingly, whichever one of the word line driving current supplying signals is selected, the word line rising time is substantially equal. Accordingly, setting of the timing of activation of the sensing amplifiers after rising of the word lines is easy.

Claims: A semiconductor memory device comprising: a memory cell array including a plurality of memory cells disposed in rows and columns, said memory cell array being divided into a plurality of blocks selected by an upper part of the row address, each block of said memory cell array being further divided into an equal number of sub blocks (SB..) in a word line direction; a plurality of word lines for selecting a row of said plurality of memory cells; a plurality of bit lines to each of which a column of said memory cell array is connected; sensing amplifiers (SA..) corresponding to each of said sub blocks (SB..) for amplifiying data of the corresponding bit lines; word line driving circuit means (WD..) corresponding to each of said sub blocks; row decoders (RD..) for selecting the word line driving circuit means (WD..) in each of said blocks; and characterized by block select signal circuit means (BSG..) for inputting all or an upper part of the row address and outputting a block select signal indicative of a block corresponding to said upper part of the row address; a plurality of word line driving current supplying circuit means (RA..) for being selectively activated in response to all or part of the upper row address for selection of one or a group of said plurality of blocks corresponding to the word line driving current supplying circuit means to supply word line driving current to said word line driving circuit means (WD..) included in said one block or said group of blocks; the word line driving circuit means arranged in the same column direction being supplied with word line driving current by way of at least two separated signal lines. The semiconductor memory device as claimed in claim 1, wherein one bit of a row address is inputted as a selection signal to said word line driving current supplying circuit means (RA11, RA12), and those of said blocks which are connected to a word line driving current supplying circuit means which is activated when said one bit is 1 and those of said blocks which are connected to another word line driving current supplying circuit means which is activated when said one bit is 0 are disposed alternately. The semiconductor memory device as claimed in claim 1, characterized in that said one block or said group of blocks is selected depending on an output of said block select signal circuit means (BSG..). The semiconductor memory device as claimed in claim 1, characterized in that each word line driving signal supply circuit means (RAG..) is selected by said block select signal and supplies a word line driving signal to word line driving circuit means (WD..) included in one block. The semiconductor memory device as claimed in claim 1, characterized in that said block select circuit means (BSG1) comprises a second block select circuit means (BSG2) for selecting word line driving supply circuit means corresponding to a group of blocks and that each word line driving signal supply circuit means (RAG..) supplies word line driving signals to word line driving circuits (WD..) included in a group of blocks.

Inventors: CHISHIKI SHIGEO, CHISHIKI, SHIGEO
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1994
Main classification: G11C8
IPC codes: G11C8, H01L21, G11C11, H01L27